A composite structure may be formed by laying up a stack of composite plies to form a composite layup. Each composite ply may include a plurality of reinforcing fibers or fiber tows. Each fiber tow may include a plurality of reinforcing filaments. For example, a single fiber tow may be formed as a bundle of several thousand or more reinforcing filaments. Heat and/or pressure may be applied to the composite layup to consolidate the stack of composite plies and cure or solidify the resin. The composite layup may be passively or actively cooled to form a composite structure.
Composite structures are typically designed to have a specific fiber volume fraction to meet strength, stiffness, and other design requirements of the composite structure. Fiber volume fraction may be described as the ratio of the total volume of reinforcing fibers in a composite structure to the total volume of the composite structure. The accuracy with which the fiber volume fraction of a composite layup can be controlled during manufacturing may have a significant effect on the ability of the composite structure to meet the design requirements.
The spacing between reinforcing filaments in a composite layup may have a direct effect on the fiber volume fraction of a composite structure. For composite plies that are pre-impregnated with resin (e.g., prepreg composite plies) prior to layup, the spacing between the reinforcing filaments may be directly controlled by controlling the amount of resin applied to the fiber tows or reinforcing filaments during prepregging operations. For composite layups formed with dry fiber composite plies, the spacing between the reinforcing filaments may be indirectly controlled by controlling the layup processing conditions such as controlling the magnitude and/or duration of consolidation pressure and/or vacuum pressure applied during infusion of resin into the dry fiber composite plies.
In conventional composites manufacturing, the local spacing between reinforcing filaments may vary from many filament diameters to zero. At zero spacing, the reinforcing filaments are in direct contact with one another. Direct contact between reinforcing filaments may result in several undesirable effects. For example, directly-contacting reinforcing filaments may inhibit resin flow between reinforcing filaments during a resin infusion process which may result in incomplete wetting of the reinforcing filaments creating undesirable voids and sites for crack nucleation. In addition, directly-contacting reinforcing filaments may represent locations of high stress in a composite structure, and may result in micro-cracking in the resin.
As can be seen, there exists a need in the art for a system and method for controlling the local spacing between reinforcing filaments in a composite layup to allow for resin flow between filaments, to meet fiber volume fraction requirements, and to reduce or avoid direct contact between reinforcing filaments.